Ink jet printing mechanisms use pens that shoot droplets of colorant onto a printable surface to generate an image. Such mechanisms may be used in a wide variety of applications, including computer printers, plotters, copiers, facsimile machines, and other printing mechanisms. For convenience, the concepts of the invention are discussed in the context of a printer. An ink jet printer typically includes a print head having a multitude of independently addressable firing units. Each firing unit includes an ink chamber connected to a common ink source via channels in a substrate, to an ink outlet nozzle or orifice defined in a thin metal orifice plate common to all nozzles on a print head. In some configurations, a three color pen has three different channels running parallel to each other and nearly spanning the entire substrate.
Ink jet print heads are susceptible to performance problems if contaminants build up on the orifice plate surface. Ink droplets may collect on the surface adjacent to the orifices, causing expelled droplets to be diverted by the presence of a droplet near one edge of the orifice. A build up of droplets may lead to puddling on the surface. If the puddling is extensive, it may provide a capillary path between nozzles of different colors, causing cross contamination or color intermixing that may extend into the ink supplies, as ink from a higher pressure supply migrates to a lower pressure supply. With extensive puddling, nozzles may become covered with ink, causing either a malformed or misdirected droplet, or preventing droplet ejection entirely. In addition, particles such as paper fibers may accumulate on the surface, partially or fully blocking a nozzle.
Accordingly, it has been customary to employ a flexible wiper to occasionally wipe across the surface of the orifice plate to remove debris and excess ink. Wipers also serve to prime firing units that are low on ink by contacting the surface of the orifice with an entrained ink film that draws ink up from the nozzle by way of capillary action. While generally effective, such wipers have several disadvantages. A wiper may serve as a vehicle to for color intermixing, as it wipes a puddle or dried ink particles from the nozzles of one color to the nozzles of another color. The orifice plate may be enlarged to reduce proximity between nozzles of different colors, but this increases the size and cost of the orifice plate. Wipers also may accrete debris or dried ink, which may further cause intermixing, and which may clog orifices or otherwise impair wiping effectiveness.
Wiping also may cause degradation of the orifice plate by the wearing action of the wiper. With non-metallic orifice plates such as those formed of polyimide (e.g. Kapton) film, the edges of an orifice may become abraded by wiping action. The edge may also become "ruffled," with flakes of material peeling slightly upward on an edge of the orifice. Any orifice wear or damage can cause droplets to be deflected from their intended path, impairing print quality.
Selection of wiper materials has traditionally faced a trade off of several factors, including wiper durability, orifice plate wear, and wiper effectiveness. For instance, a harder wiper material may provide high local pressures for effective scraping of contaminants, but at the cost of increased orifice plate wear. A soft material may not cause wear, but may be susceptible to wear that degrades wiping performance over time.
For efficient ink jet printing without excessive energy consumption, the volume of ink in the firing chamber should be minimized, reducing the ink mass to be moved upon firing, thus creating a more responsive firing characteristic. One factor affecting this volume is the height of the firing chamber, defined by the distance between the resistor film at the base of the chamber and the upper surface of the orifice plate, which normally defines the upper edge of the nozzle. To reduce volume by reducing plate thickness has the disadvantage of weakening plate strength and rigidity, making assembly more difficult, and potentially impairing reliability.
Therefore, there exists a need for an ink jet print head that overcomes or reduces at least some of these disadvantages. The disclosed embodiments address this need by providing an orifice plate for a thermal ink jet print head. The plate has a plurality of orifice apertures, with a major surface occupying a first plane. The plate has a surrounding region surrounding each of the orifices, and the surrounding region has an offset portion with an offset surface offset from the first plane. The offset portion may be above or below the first plane, and may include concentric inner and outer regions, with the outer region above the first plane, and the inner region recessed below the outer region.